Whenever two movable members are to be displaced simultaneously between two equal positions on separate trajectories, the natural thing to do is to have them mechanically connected and to drive them together as a unit. However, there are instances where each member must be driven separately. Such an instance is the screwdown system of a rolling mill. In a screwdown system two screw mechanisms are applied at opposite ends of a workroll to accurately position the rolls at a definite gauge for the workpiece to be rolled. In such a case, the prior art makes use of worm gears connected to the screws and a clutch arrangement tying the screws together for balancing out the torques developed. The screws must accurately track one another or they will bind. They must also stop at exact equal positions if the mill roll is maintained level at the required gauge. Accurate tracking and stopping is particularly important when there is automatic gauge control on the mill. Automatic gauge control can be used not only to set the screwdown system but also to correct against workroll deformation, or even to provide controlled bending to shape the workroll under the right and left screwdown mechanism. An illustration of a screwdown system using a clutch between the motor drive of the screws can be found in U.S. Pat. No. 3,096,670 of C. F. Stringer. The same problem is encountered when controlling movement of the edges, or of the sideguides in a rolling mill and it has received a similar solution. In such a case the edges, or the side-guides, are moved laterally toward the pass line into a position of contact with the workpiece, from opposite directions relative to the central line of the workstands. Along these directions they are both retracted, or applied, at the same time.
The modern sophistication of electrical control makes it attractive to do without a clutch between the two drives and to rely on electrical circuitry only in order to create an electrical match between the two drives and the two driven members at all times during the control operation. An example of separate motor drives with no mechanical connection therebetween is found in U.S. Pat. No. 3,789,280 which shows a hoisting system for a conveyance held by several ropes. There, the tension of the ropes has to be kept equalized while lifting or lowering the conveyance under the separate motor drives. A master-slave system is used and any unbalance in the torques serves to compensate for it by controlling the slave motor drive so as to follow the master motor drive. However, such a system, because it relies only on the torques, does not impose an accurate relation between the displacements due to the respective motor drives. The problem there is somewhat different from a situation in which accurate positioning is an important as a good tracking between the movable members.
One object of the present invention is to provide an electrical shaft connecting two individually driven members which are to be moved together.
Another object of the present invention is to provide an improved screwdown system for a rolling mill which accurately controls in position two self-tracking screw mechanisms relative to the workroll.
A further object of the present invention is an improved system for imparting correlative displacements to at least two members moving along a separate trajectories under separate motor drives an without mechanical interconnection.